Capacitive sensor for non-contacting gap and dielectric medium measurement

ABSTRACT

A non-contact capacitive sensor including: a sensor plate configured to be displaced from a surface and to measure a capacitance of a gap between the surface and sensor plate; an active shield plate over the sensor plate and insulated from said sensor plate, wherein a high frequency input signal is applied to the active shield plate and sensor plate; an effective ground shield plate connected through a first resistor to a ground, over the active shield plate to sandwich the active shield plate between the ground shield plate and the sensor plate, and the ground shield plate is insulated from the active shield plate, and a second resistor connected between the ground shield plate and the active shield plate to provide a direct current (dc) path through the sensor.

CROSS RELATED APPLICATION

This application is a divisional of and claims priority to U.S.application Ser. No. 10/825,185 filed on Apr. 16, 2004 and isincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a method and system for non-contactmeasurement of a gap between a sensor and a conductive or non-conductivesurface using a capacitive measurement device with a pluralityconductive plates that permits measurement of material depth anddielectric changes in solids and fluids.

Non-contact gap measurement sensors having two parallel superimposedconductive plates, which are electrically insulated from one another,are disclosed in, for example, U.S. Pat. Nos. 4,675,670; 5,990,807;6,075,464 and 6,552,667. A high frequency signal is placed on the firstplate of the sensor. By measuring the capacitance between the sensor anda proximate surface, the sensor generates a signal that is indicative ofthe gap between the sensor and the surface.

A difficulty with existing non-contact capacitive sensors is that thecapacitive signal generated by the sensor that is representative of thegap may be overshadowed by noise. The noise may arise from capacitancevariations of extension cables between the sensor and its associatedelectronics, signal pickup electronics and transformer, and straycapacitances from the signal pickups. The noise due to these capacitancevariations may be much greater than the capacitance of the signalindicative of the gap.

Another difficulty in using a capacitive sensor is that capacitance isformed between the sensor and any surfaces which come near the sensor,including surfaces behind the sensor. Some sensitivity to surfacesbehind the sensor remains even when an active shield plate is placedbehind the sensor plate.

There is a need for a capacitive measurement method and a non-contactcapacitive measurement sensor that is less sensitive to variations incapacitance, in an insulator between the two plates of the sensor andvariations in the impedance of the cables connecting the sensor to thecircuit. Excessive sensitivity to these variations may increase thedifficulty in manufacturing the sensor and increase the sensorsensitivity to temperature and other environmental factors.

BRIEF DESCRIPTION OF THE INVENTION

An electronic circuit which directly measures the capacitance of asensor relative to a surface or dielectric medium by having thecapacitance change the voltage gain of an amplifier and which is used toprovide a non contacting gap measurement. Also, a capacitive sensorhaving three parallel superimposed conductive plates, with a sensorplate which is electrically insulated from the other plates, withgreatly reduced sensitivity to surfaces behind the sensor.

The invention may be embodied as a method for non-contact measurement ofa displacement between a surface and a capacitive sensor comprised of atleast two superimposed conductive plates electrically insulated one fromthe other and a sensor circuit coupled to the plates including:positioning the capacitive sensor proximate to the surface such that thedisplacement is a distance of a gap between the surface and one of theplates; applying a high frequency signal to the plates; applying thehigh frequency signal and a signal from a sensor plate of the conductiveplates to control a voltage gain of an amplifier in the circuit, wherethe applied sensor signal is indicative of the displacement between thesensor and surface; differentiating an output of the amplifier and thehigh frequency signal, and determining a value of the displacement basedon the difference between the output of the amplifier and the highfrequency signal.

The invention may also be embodied as a method for non-contactmeasurement of a displacement between a surface and a capacitive sensorcomprised of at least three superimposed conductive plates electricallyinsulated from each other and a sensor circuit coupled to the plates,wherein said plates include a sensor plate, an active shield platesandwiched between a sensor plate and a passive shield plate, saidmethod comprising: (a) positioning the capacitive sensor proximate tothe surface such that the displacement is a distance of a gap betweenthe surface and the sensor plate; (b) applying a high frequency signalto the sensor plate and to the active shield plate; (c) applying asignal induced on the sensor circuit by the high frequency signal andthe sensor plate to control a voltage gain of an amplifier in thecircuit, said applied sensor signal being indicative of the displacementbetween the sensor and surface; (d) differentiating the output of theamplifier and the high frequency signal, and (e) determining a value ofthe displacement based on the difference between the applied signal andthe high frequency signal.

The invention may also be embodied as a non-contact capacitive sensorcomprising: a sensor plate which is configured to be displaced from asurface to measure a capacitance of a gap between the surface and sensorplate; an active shield plate over said sensor plate and insulated fromsaid sensor plate, wherein a high frequency input signal is applied tothe active shield plate and sensor plate; an effective ground shieldplate connected through a first resistor to a ground, over said activeshield plate so as to sandwich the active shield plate between theground shield plate and the sensor plate and said ground shield plate isinsulated from the active shield plate; a second resistor connectedbetween the passive shield and the active shield providing a dc paththrough the sensor.

The invention may be further embodied as a method for non-contactmeasurement of a dielectric related characteristic of a medium between asurface and a capacitive sensor comprised of at least two superimposedconductive plates electrically insulated one from the other and a sensorcircuit coupled to the plates, said method comprising: positioning saidcapacitive sensor proximate to the surface such that the medium isbetween the surface and a sensor plate of the plates; applying a highfrequency signal to the plates and a dielectric of the medium affects aresponse signal of the sensor plate to the high frequency signal;applying the high frequency signal and the response signal from thesensor plate to control a voltage gain of an amplifier in the circuit,said response signal being indicative of the medium between the sensorand surface; differentiating an output of the amplifier and the highfrequency signal, and determining a value of the dielectric based on thedifference between the output of the amplifier and the high frequencysignal.

The invention may also be embodied as a method for non-contactmeasurement of a medium proximate to a capacitive sensor comprised of atleast two superimposed conductive plates electrically insulated one fromthe other and a sensor circuit coupled to the plates, said methodcomprising: positioning said capacitive sensor proximate to the medium;applying a high frequency signal to the plates; applying the highfrequency signal and a signal from a sensor plate of the conductiveplates to control a voltage gain of an amplifier in the circuit, saidsignal from the sensor plate being indicative of a property of themedium; differentiating an output of the amplifier and the highfrequency signal, and determining a value of the property of the mediumbased on the difference between the output of the amplifier and the highfrequency signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic diagrams of a non-contacting capacitivesensor. FIG. 2 is an enlarged view of an end section of the sensor and asurface shown in FIG. 1.

FIG. 3 is a schematic diagram of an electronic circuit associated withthe capacitive sensor.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show schematically a sensor 10 near a surface 12 and a gap14 between the sensor and surface. The sensor generates a signalindicative of the distance of the gap or of a proportionality of adielectric medium in front of the sensor. In addition to measuring adistance of a gap, the sensor may also be used to determine a change ina dielectric of a fluid flowing in front of the sensor, or the thicknessof a material.

The sensor 10 comprises three adjacent conductive plates 16, 18, and 19that are electrically isolated from each other. The second (activeshield) plate 16 shields the first (sensor) plate 18 from surfacesbehind the sensor, and from the third (passive shield) plate 19. Thesensor plate 18 faces the surface 12 and the gap. The sensor plate isused to measure the capacitance across the gap and is oriented parallelto the surface. The active shield plate 16 is immediately behind thesensor plate 18 and actively shields the sensor plate by being connectedas an input to an operational amplifier 20 that also has an input fromthe sensor plate 18.

In addition to measuring a gap displacement, the sensor 10 may also beapplied to measure a depth of a fluid and the thickness of a material.The capacitance 23 signal from the sensor plate 18 is influenced by thedielectric of the adjacent medium. The adjacent medium may be, forexample, an air gap between the sensor plate and another surface 12, afluid across the sensor plate or a solid material abutting the sensorplate. The dielectric of the adjacent medium effects the capacitance 23which in turn effects the signal from the sensor plate. The dielectricof the medium adjacent the sensor plate may be indicative of: a depth ofor impurities in a fluid—where the fluid is the medium, or the thicknessof or impurities in a solid—where the solid is the medium. Accordingly,the sensor may be used to measure the depth of a fluid, the thickness ofa solid medium or impurities in a medium adjacent the sensor plate.

The passive shield plate 19 provides additional shielding from surfacesbehind the sensor. As shown in FIG. 3, the passive shield plate 19 ofthe sensor is connected to ground through a resistor 17 which is mountedon the sensor. The resistor value may be between 0 ohms and 10000 ohms(10K Ω). The resistor 17 value maybe selected such that the maximumamount of shielding is obtained from surfaces behind the sensor. Thepassive shield 19 is also connected to the active shield plate 18through a resistor 21 to provide a dc current path for a not-OK circuit36 to detect an opened or shorted connector. A short between the passiveand active shield plates 18, 19 will generate a dc voltage on the inputline 28 to the op-amp 20 and also to the not-OK circuit 36. Upondetecting the dc voltage, the not-OK circuit 36 disables the outputdriver 50 and the output signal 42 of the sensor circuit.

FIG. 3 is a schematic diagram of an electronic circuit for the sensor10. The circuit includes an operational amplifier (op-amp) 20 having acapacitive feedback loop 22 to detect the capacitance 23 of the gap 14between the sensor 10 and the surface 12. A high frequency signal 24 isapplied to a non-inverting input 28 of the op-amp 20 and to the activeshield plate 16 of the sensor. The high frequency signal may be between1 kHz and 10 MHz, and have a non-varying peak-to-peak voltage of between1 to 100 volts. The cyclical signal 24 is output by a high frequencygenerator or oscillator circuit 26 which provides a fixed amplitude acsignal.

The capacitance between the sensor plate 18 and the surface 12 sets thehigh frequency gain (output voltage change/input voltage change) of theop-amp 20. The capacitance 23 between the sensor and the surface 12varies as a function of gap 14. The voltage gain of the op-amp 20likewise varies as a function of the gap 14. A constant high frequencysignal 24 is applied to the non-inverting input 28 and a signal outputfrom the sensor plate 18 is applied to the inverting input 30. Becausethe high frequency signal 28 has a constant amplitude and the voltagegain of the op-amp 20 varies as a function of the gap capacitance 23,the output voltage of the op amp changes as a function of the gap.

The inverting input 30 of the op-amp is connected to the sensor plate18. The op-amp 20 maintains the sensor plate signal applied to theinverting input 30 equal to the high frequency signal 24 applied to thenon-inverting input 28 and to the active shield plate 16. Because thesignal is equal at both op-amp inputs 28, 30, the impedance between thesensor plate and the active shield plate is not a part of themeasurement. The capacitance and impedance variations which do occurbetween the two inputs, between the plates of the sensor and between theconductors in the cable connecting the circuit to the sensor aresubstantially eliminated.

The output signal 32 (Output) from the op-amp 20 is equal to:Output=Vin+Vin×C(measurement)/C(Feedback)

Vin is the high frequency signal applied to the non-inverting input 28;C(measurement) is the capacitance between the sensor plate 18 and thesurface 12, and C(feedback) 52 is the capacitance between the output 32of the op-amp and the inverting input 30 of the op-amp. C(measurement)is the capacitive value 23 that is to be measured and is indicative ofthe gap 14 distance.

The difference between the high frequency signal 24 and the op-ampoutput signal 32 is indicative of the capacitance 23 of the gap 14between the sensor plate 18 and surface 12. The op-amp output signal 32and high frequency signal 24 are applied to a differential amplifier 34that generates an oscillating voltage difference signal 37 indicative ofthe gain applied by the op-amp to the input signal 24 which in turn isindicative of the capacitance of the gap.

The voltage difference signal 37 is demodulated from the high frequencyinput signal using demodulator 38, e.g., a peak detector, and linearized40. The final output voltage 42 is proportional to the gap 14 beingmeasured. The measured capacitance (C(measurement)) 23 is inverselyproportional to the gap. The linearizer 40 provides an output voltage 46which is inversely proportional to the input voltage. An output driver50 outputs 42 the output voltage 48 unless the non-OK circuit 36 hasdisabled the driver.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A non-contact capacitive sensor comprising: a sensor plate configuredto be displaced from a surface and to measure a capacitance of a gapbetween the surface and sensor plate; an active shield plate over saidsensor plate and insulated from said sensor plate, wherein a highfrequency input signal is applied to the active shield plate and sensorplate; an effective ground shield plate connected through a firstresistor to a ground, arranged over said active shield plate to sandwichthe active shield plate between the ground shield plate and the sensorplate, and insulated from the active shield plate, and a second resistorconnected between the ground shield plate and the active shield plate toprovide a direct current (dc) path through the sensor.
 2. Thenon-contact capacitive sensor of claim 1 wherein said plates aresuperimposed.
 3. The non-contact capacitive sensor of claim 1 whereinsaid plates are laminated together with insulation between each plate.4. The non-contact capacitive sensor of claim 1 further comprising acapacitance between the active plate and sensor plate, and a secondcapacitance between the active shield plate and the passive shieldplate.
 5. The non-contact capacitive sensor of claim 1 wherein saidsensor plate has a planar surface facing the surface.
 6. The non-contactcapacitive sensor of claim 1 wherein the sensor plate and active shieldplate further comprise connections to a sensor circuit.
 7. A non-contactcapacitive sensor and sensor circuit assembly comprising: a sensor plateconfigured to be displaced from a surface and to measure a capacitanceof a gap between the surface and sensor plate; an active shield plateover said sensor plate and insulated from said sensor plate, wherein ahigh frequency input signal is applied to the active shield plate andsensor plate; an effective ground shield plate connected through a firstresistor to a ground, arranged over said active shield plate to sandwichthe active shield plate between the ground shield plate and the sensorplate, and insulated from the active shield plate; a second resistorconnected between the ground shield plate and the active shield plate toprovide a direct current (dc) path through the sensor, and the sensorcircuit receiving as an input a signal from of the sensor plate andgenerating an output indicative of the gap.
 8. The non-contactcapacitive sensor and sensor circuit assembly of claim 7 wherein thesensor circuit further comprises an amplifier sensing a differencebetween an output of the amplifier and the high frequency input signal.9. The non-contact capacitive sensor and sensor circuit assembly ofclaim 7 wherein the sensor circuit further comprises an operationalamplifier receiving as inputs a high frequency output of the sensorplate and receiving a feedback signal comprising an output of theoperational amplifier.
 10. The non-contact capacitive sensor and sensorcircuit assembly of claim 9 wherein the sensor circuit assembly furthercomprises a difference amplifier receiving the output of the operationalamplifier and a peak detector wherein the operational amplifier includesapplying the output of the amplifier as feedback to the signal from thesensor plate.
 11. The non-contact capacitive sensor and sensor circuitassembly of claim 7 wherein said plates are superimposed.
 12. Thenon-contact capacitive sensor and sensor circuit assembly of claim 7wherein said plates are laminated together with insulation between eachplate.
 13. The non-contact capacitive sensor and sensor circuit assemblyof claim 7 further comprising a capacitance between the active plate andsensor plate, and a second capacitance between the active shield plateand the passive shield plate.
 14. The non-contact capacitive sensor andsensor circuit assembly of claim 7 wherein said sensor plate has aplanar surface facing the surface.
 15. The non-contact capacitive sensorand sensor circuit assembly of claim 7 wherein the sensor plate andactive shield plate further comprise connections to a sensor circuit.